The present invention relates to a method and a corresponding device for diagnosing the operability of fuel vapor intermediate stores, in particular activated carbon filters, in tank venting systems. Furthermore the present invention is subject to a computer program and a computer program product, which qualify for the implementation of the method.
Fuel vapors occur from the fuel container or tank at motor vehicles. In order to avoid an emission of these volatile hydrocarbons from the tank in particular at motor vehicles with Otto-engines or to limit it, usually devices for catching the fuel vapors, so-called fuel vapor intermediate stores, are provided. An activated carbon filter (AKF) or an activated carbon container is usually provided for this purpose. The ventilation line of the fuel container runs into this activated carbon filter. A further line goes from the activated carbon filter to the inlet manifold of the engine. A tank venting valve (regenerating valve) is generally provided in this line.
The activated carbon absorbs the fuel or the volatile hydrocarbons that are contained in the fuel vapor. For regenerating or flushing the activated carbon filter the tank venting valve is opened, so that the line between the activated carbon container and the inlet manifold is free. Due to the negative pressure that exists in the inlet manifold fresh air is sucked in through the activated carbon. The fresh air takes in the absorbed fuel in the flush current and supplies it to the combustion in the engine. By using a regular regeneration of the activated carbon the activated carbon filter remains absorbable for new evaporating fuel.
The fuel vapor intermediate store or the activated carbon filter is a central emission-relevant component in the tank system. The activated carbon filter allows to limit emissions, which are caused by the evaporation of fuel or gasoline from the fuel container or to comply with statutory emission thresholds for evaporation losses. Therefore an operative activated carbon filter is required with a corresponding storage capacity.
But there is the problem that an activated carbon filter can have an impaired operability. The activated carbon filter can for example be flooded with liquids, in particular liquid hydrocarbons or fuel. Thereby the absorption capacity for volatile hydrocarbons sinks drastically and liquid fuel might leak. A regeneration of a partially or completely flooded activated carbon filter requires a long period of time. During this period of time the operability is strongly limited. Furthermore the activated carbon can be partially or completely overlaid by nonvolatile parts of the fuel. The storage capacity of the activated carbon filter sinks then significantly and the operability of the activated carbon filter is limited permanently. In other cases the activated carbon can be damaged for example by a mechanical load, for example by commotions, so that a reduction of the storage capacity also occurs here as well as a limitation of the operability of the activated carbon filter.
Thus there is the need to control the operability of the fuel vapor intermediate store or the activated carbon filter and to carry out a diagnosis of the activated carbon filter.
Different methods are already known, which determine the load status of an activated carbon filter. This determination takes place with the aim to optimize the regeneration of the activated carbon in particular with regard to the frequency of the regeneration runs. The published patent application DE 199 35 886 A1 describes for example a control system for the vapor recirculation at engines with a direct injection. The flushing of the activated carbon filter or the vapor recirculation is usually not possible at engines with direct injection during the operation in shift-mode. Therefore the engine has to work in homogenous mode in regular intervals in order to carry out the flushing. The operation in shift-mode, which is advantageous with regard to the fuel consumption, is limited by the required flushing of the activated carbon container. Therefore the load status of the activated carbon filter is determined according to DE 199 35 886 A1 in order to minimize the frequency of the flushing processes. Therefore a vapor sensor container that is filled with activated carbon is equipped with temperature sensors, which allow a determination of the hydrocarbon concentration in the vapor recirculation system by measured temperature differences.
The publication of the Japanese patent application JP 2004 35 35 55 A describes the use of weight sensors, vapor pressure sensors and vapor temperature sensors, in order to check the absorption status in an activated carbon filter.
The international publication WO2004/083619 A1 also describes the use of temperature sensors in order to determine the degree of saturation of an activated carbon filter.
With these familiar methods statements about the load status of the activated carbon filter in the tank venting system of a motor vehicle can be made. But it is not possible to check the operability of the activated carbon filter with these methods. It can for example not be checked with these methods, whether parts of the activated carbon are permanently overlaid with nonvolatile parts of fuel, whereby the operability of the activated carbon filter is limited in the long run. The invention has therefore the task to provide a procedure for checking or diagnosing the operability of fuel vapor intermediate stores in tank venting systems. By ensuring an operative fuel vapor intermediate store it can be guaranteed that the environment is protected from fuel vapor emissions. Additionally the diagnosis of the fuel vapor intermediate store, being an emission-relevant component, shall be enabled, as it might become a statutory requirement in the future.